Blood pressure is the most frequently measured quantity in medical physiological practice. The methods of measuring systemic blood pressure are either invasive or noninvasive. The most commonly known noninvasive method (Riva-Rocci Method) uses a cuff and sphygmomanometer to determine the maxima and minima of pressure commonly referred to as the systolic and diastolic pressure. Interpretation of the cessation and reappearance of Korotkow sounds for estimating systolic and diastolic pressure values is very subjective and improved methods become necessary is more information is to be extracted from the pressure pulsations.
Invasive methods provide more information but require that the blood vessel be punctured. Using this method, the transducer is placed inside the vessel or outside the vessel and connected to it via tubing. For true fidelity of the pressure pulse, the transducer should be as close to the puncture as possible using the shortest length of tubing. Therefore, the transducer must be small and should have a stiff diaphragm and be ruggedly constructed.
Transducers capable of measuring blood pressure are usually capable of converting pressure energy, i.e. force per unit area, into a proportional displacement. This displacement in turn can be made to vary an inductance, a capacitance or a resistance. The inductive and resistive type transducers are relatively unstable with temperature, and, moreover, the inductive type is sensitive to vibrations. While the capacitive transducer is generally difficult to produce, it is very rugged and reliable.